Electrostatic discharge (ESD) is a sudden, usually unwanted, current that can flow between objects, such as electronic circuits or devices, at different potentials due to the discharge of static charge of an object. ESD events can, for example, be caused by static electricity, for instance due to tribocharging. Another cause of ESD is electrostatic induction, which can occur when an electrically charged object is placed near a conductive object isolated from ground. The charged object causes an electrostatic field. This electrostatic field can then cause a redistribution of electrical charges on the surface of the conductive object.
ESD typically occurs during a short period of time and often causes a high current during this period of time which can be referred to as an ESD stress event. Thus, ESD can result in immediate, permanent damage of the electronic circuit or can cause performance degradation of the circuit.
Electronic circuits are typically required to sustain, at least to a certain extent, ESD stress events without damage to the electronic circuit. ESD sustainability of an electronic circuit can be measured according to standards dedicated to an intended field of application of the electronic circuit. ISO10605 by the International Organization for Standardization, for example, is a standard dedicated mainly to road vehicles, that defines stress test methods for electrical disturbances from electrostatic discharge, which are applicable to automotive electronic modules and vehicles. IEC61000-4-2 by the International Electrotechnical Commission is another example of standard that describes testing and measurement techniques of devices submitted to electrostatic discharge immunity test, that are applicable to electrical or electronic equipment.
To reduce or avoid a negative impact of the ESD current on the electronic circuit, ESD protection circuits can be used. The ESD protection circuit can be designed to sustain the ESD event without damage, or to be damaged instead of the electronic circuit protected by the ESD protection circuit. ESD protection circuits can for example, at least partially, absorb the ESD current. This can, for example, be achieved by diverting the ESD current away from the ESD sensitive circuits, e.g., into ground or the substrate, and/or by transforming electrical energy of the discharge into, for example, thermal energy by the protection circuit.
Another way of absorbing is, for example, to transform the energy into field energy by loading a capacitor. An electrostatic discharge between two pins, i.e. two input/output terminals of the electronic circuit, can, for example, be absorbed by a large external capacitor connected between the pins. However, this requires an additional external discrete element to be connected to the pins.